You also have to get a lot of paperwork together, that you usually don't have and it can take months as the company drags it's feet in giving it to you. There wasn't paperwork preventing this at the company I was at.
If anyone is thinking about a startup offer, I would suggest getting equityzen's documentation checklist upfront. It can take longer than you think, and a few months later, people who were interested in buying might not be anymore.
I was trying this a little less than a year ago, so it isn't as correlated to the current issues today.
Also I would try to get the stock paperwork agreement that they will have you sign for your options. Very surprisingly, for even some big superstars today it is very hard to get this from them until you start working for them. There can be hidden gotchas like after exercise stock buyback options at the current FMV. Or restrictions that even stop you from doing things like ESO fund.